Experimental and Numerical Study of Parallel Flow Evacuated U-Tube Solar Collector With Parabolic Reflector: Impact of Particulate Matter

The article explores the performance of an evacuated U-tube solar collector integrated with a parabolic reflector under extremely hot tropical conditions, both with and without the presence of particulate matter, using experimental and simulation approaches. Experimental results showed that at peak conditions of 1017-W/m² solar intensity and 42 ℃ ambient temperature, the heat transfer fluid (HF) exhibited a temperature increase of 11.5 ℃, accompanied by a heat gain of 1100 W, with a flowrate ranging from 0.028 to 0.03 L/s and a U-tube contact area of nearly 0.18 m². In these tropical conditions, the maximum thermal efficiency achieved was 66% without particulate matter and 61% with particulate matter treatment (PMT), which simulated the accumulation of particles on the evacuated tube's surface. The introduction of PMT on the evacuated tube's outer surface led to slight deteriorations, including a 2 ℃ reduction in HF temperature increase, a 200-W decrease in heat gain, and a 17% drop in thermal efficiency compared to the scenario without PMT. The study also includes case studies using two numerical models to assess the time required to reach a steady state and to understand the system's thermal behavior throughout the day, both with and without PMT. The analyses reveal that under peak solar conditions, a steady state is achieved in 117 s with PMT and 131 s without, at a flowrate between 0.026 and 0.028 L/s. Additionally, the impact of HF flowrate, solar intensity, and HF inlet temperature on thermal performance is examined, revealing intricate temperature patterns along the U-tube's radial and axial directions. Detailed 3D temperature contours for hourly variations on sunny days and transient analyses along the collector length are presented. These findings offer valuable insights for optimizing solar collector systems for extremely hot tropical climates.